Many types of machines have moveable members which are operated by a hydraulic cylinder that is supplied with fluid under pressure to move a piston within the cylinder. A valve controls application of the fluid to the cylinder. In a typical machine a combination of valves control multiple functions, such as raising and lowering a boom of a backhoe, tilting or rotating the boom, and sliding members fore and aft. By varying the degree to which the valve is opened, the flow rate of the hydraulic fluid can be varied thereby moving the piston at proportional speeds. The Wilke U.S. Pat. No. 3,469,272, discloses a basic reciprocal spool valve for controlling the flow of the hydraulic fluid.
Typically the valve is operated manually by a lever that was mechanically connected to the spool. However a present trend is away from using manually operated hydraulic valves toward electrically controlled solenoid valves. Solenoid valves are well known electromagnetic actuated devices for controlling the fluid flow. A solenoid valve involves an electromagnetic coil which moves an armature in one direction to open a valve. The amount that the solenoid valve is opened is controlled by varying the electric current applied to the solenoid coil. Either the armature or a valve member is spring loaded so that the valve closes when the electric current is removed from the solenoid coil.
With either mechanically, electrically, or hydraulic pilot operated valves it often is desirable to sense the position of the spool and provide an electrical signal which indicates that position. Such sensing must not add excessive load to the movement of the valve spool.